1. Field of the Invention
The present invention relates to an optical fiber switch employed for an optical fiber communications system. More particularly, the invention relates to a 2.times.2 optical fiber switch that is provided with four optical fiber assemblies equipped with lenses, in which collimator lenses are matched to optical fibers, and that employs prisms to shift two optical fiber circuits in parallel to or across another two optical fiber circuits so as to switch the connection of the optical fiber circuits.
2. Description of the Related Art
There has been known a 2.times.2 optical fiber switch constituted using collimator lenses and prisms disclosed, for example, in U.S. Pat. No. 5,436,986. FIGS. 4A and 4B are schematic top plan views illustrating the operation of such a 2.times.2 optical fiber switch. An optical fiber 102 and a sleeve 103 are inserted in the central hole of a ferrule 101 to constitute a ferrule assembly with an optical fiber. A collimator lens 104 is provided coaxially on an end surface of the ferrule assembly with the optical fiber. Thus, four optical fiber lens assemblies 100A, 100B, 100C, and 100D are fabricated. Two each of these four optical fiber lens assemblies 100A through 100D are mounted on holders (not shown) such that they are coaxial and parallel to each other. Rectangular prisms 105 and 106 have short side surfaces 107 through 110 thereof provided with deposited reflection coat layer made of metal or the like.
The respective rectangular prisms 105 and 106 are secured to a holder plate 111. The rectangular prisms 105 and 106 are installed so that long sides 112 and 113 are parallel and rectangular intersection points 117 and 118 are located symmetrically. The holder plate 111 is engaged with a solenoid coil magnet which is not shown. If the direction of the axes of the optical fiber lens assemblies 100A through 100D is denoted by (Z), then the holder plate 111 is constructed so that it reciprocates in the direction of an axis (X) orthogonal to the optical axes (Z) when the current polarity of the solenoid coil magnet is switched.
FIG. 4A shows the holder plate Ill when it has advanced. In this state, a parallel beam emitted through the collimator lens of a left optical fiber lens assembly A is reflected by the reflection coat layer of the short side surface 108 of the rectangular prism 105 and projected onto the reflection coat layer of the short side surface 110 of the rectangular prism 106 as indicated by the white arrows. The projected parallel beam is further reflected by the reflection coat layer of the short side surface 110 of the rectangular prism 106 and enters the optical fiber of a left optical fiber lens assembly B.
Likewise, a parallel beam emitted through the collimator lens of a right optical fiber lens assembly D is reflected by the reflection coat layer of the short side surface 109 of the rectangular prism 106 and projected onto the reflection coat layer of the short side surface 107 of the rectangular prism 105 as indicated by the black arrows. The projected parallel beam is further reflected by the reflection coat layer of the short side surface 107 of the rectangular prism 105 and enters the optical fiber of a right optical fiber lens assembly C.
The 2.times.2 optical fiber switch constituted by using the conventional collimator lenses and prisms works as set forth above. With the holder plate 111 of FIG. 4A in the advanced position, the position of a central axis 114 of the left optical fiber lens assemblies A and B must be accurately aligned with a symmetry central line position 115 of the rectangular prisms 105 and 106. As shown in FIG. 5, if an alignment error of e.sub.1 is produced between the position of the central axis 114 of the left optical fiber lens assemblies A and B and the symmetry central line position 115 of the rectangular prisms 105 and 106, then the central axis of the light beam incident on the optical fiber lens assembly B is decentered from the optical fiber axis of the optical fiber lens assembly B by e.sub.2 =2e.sub.1.
FIG. 6 shows the values obtained by the experiments carried out by the assignee on the relationship between, decentering e.sub.2 of the optical axis and insertion loss when a collimator lens having a diameter of 2 mm was used. As shown in FIG. 6, the insertion loss increases as the decentering e.sub.2 of the optical axis increases. For instance, if e.sub.1 is 25 .mu.m, then e.sub.2 will be 50 .mu.m and the insertion loss will be about 0.37 dB. Hitherto, therefore, assembling adjustment skill for controlling, to a minimum, the alignment error of e.sub.1 produced between the position of the central axis 114 of the optical fiber lens assemblies A and B and the symmetry central line position 115 of the rectangular prisms 105 and 106, or assembling adjustment skill for precisely position and fix the rectangular prisms 105 and 106 on the holder plate 111, or other similar skill has been necessary.